U.S. patent application number 11/414606 was filed with the patent office on 2007-11-01 for methods and apparatus for mri shim elements.
This patent application is currently assigned to General Electric Company. Invention is credited to Yuri Lvovsky.
Application Number | 20070252598 11/414606 |
Document ID | / |
Family ID | 38647735 |
Filed Date | 2007-11-01 |
United States Patent
Application |
20070252598 |
Kind Code |
A1 |
Lvovsky; Yuri |
November 1, 2007 |
Methods and apparatus for MRI shim elements
Abstract
A MRI shim element includes a plurality of thin wires extending
substantially parallel to each other wherein each wire has cross
sectional dimensions less than .delta..sub.sk, wherein
.delta..sub.sk is the wire's skin depth.
Inventors: |
Lvovsky; Yuri; (Florence,
SC) |
Correspondence
Address: |
FISHER PATENT GROUP, LLC
700 6TH STREET NW
HICKORY
NC
28601
US
|
Assignee: |
General Electric Company
|
Family ID: |
38647735 |
Appl. No.: |
11/414606 |
Filed: |
April 28, 2006 |
Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/3873
20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01V 3/00 20060101
G01V003/00 |
Claims
1. A MRI shim element comprising: a plurality of thin wires
extending substantially parallel to each other wherein each wire
has cross sectional dimensions less than .delta..sub.sk, wherein
.delta..sub.sk is the wire's skin depth with typical MRI
frequencies.
2. A MRI shim element in accordance with claim 1 wherein said wires
are stacked in multiple layers.
3. A MRI shim element in accordance with claim 2 wherein said wires
are round and coated with a varnish.
4. A MRI shim element in accordance with claim 2 wherein said wires
are impregnated in non-conductive bonding agent.
5. A MRI shim element in accordance with claim 2 wherein said wires
are rectangular and separated from each other by an insulating
material.
6. A MRI shim element in accordance with claim 5 wherein said
insulating material comprises a glass cloth.
7. A MRI shim element in accordance with claim 6 wherein said wires
are impregnated in an epoxy.
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. An imaging apparatus for producing Magnetic Resonance (MR)
images of a subject having a magnet assembly for producing a static
magnetic field B0 and a gradient coil assembly disposed within said
magnet assembly for generating a magnetic field gradient for use in
producing MR images, said apparatus comprising a MRI shim element
comprising a plurality of thin wires extending substantially
parallel to B0.
16. An imaging apparatus in accordance with claim 15 wherein said
wires are round and coated with a varnish.
17. An imaging apparatus in accordance with claim 16 wherein said
wires are fabricated from a low carbon steel and have a diameter of
less than about 3 mm.
18. An imaging apparatus in accordance with claim 15 wherein said
wires are rectangular with all cross sectional dimensions less than
about 3 mm.
19. An imaging apparatus in accordance with claim 18 wherein said
wires are rectangular and separated from each other by an
insulating material.
20. An imaging apparatus for producing Magnetic Resonance (MR)
images of a subject having a magnet assembly for producing a static
magnetic field B0 and a gradient coil assembly disposed within said
magnet assembly for generating a magnetic field gradient for use in
producing MR images, said apparatus comprising a MRI shim element
comprising a plurality of thin wires extending substantially
perpendicular to B0.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to methods and apparatus
for magnetic resonance imaging (MRI) systems, and more particularly
to methods and apparatus that facilitate making and using passive
shim elements that provide for low eddy currents and an improved
image stability.
[0002] Achieving a high final field homogeneity in MRI magnets
typically requires the use of magnetic shimming, either a fully
passive or a hybrid system that includes passive shims as an
integral part. Shim elements made of magnetized steel, loaded
strategically on shim rails, inserted inside the bore and saturated
by the main field of the magnet, compensate for manufacturing
tolerances and environmental inhomogeneities. These shim elements
are exposed to pulsing fields from a plurality of gradient coils
which generate heat in the shim elements, raising their temperature
and thus affecting magnetic field stability (both B0 and
homogeneity) due to the temperature-dependent saturation B.sub.s(T)
of the shims. Among major contributors to the shim heating are eddy
currents induced in the conducting shims by the changing magnetic
flux from the gradient coils. There is a need for efficient
inexpensive passive shim elements with low eddy current heat
generation.
BRIEF DESCRIPTION OF THE INVENTION
[0003] In one aspect, a MRI shim element includes a plurality of
thin wires extending substantially parallel to each other wherein
each wire has cross sectional dimensions less than .delta..sub.sk,
wherein .delta..sub.sk is the wire's skin depth.
[0004] In another aspect, a method for making a plurality of shim
elements is provided. The method includes placing a plurality of
thin wires extending substantially parallel to each other to form
either a flat sheet or an arced structure wherein each wire has
cross sectional dimensions less than .delta..sub.sk, wherein
.delta..sub.sk is the wire's skin depth, and cutting or punching
the flat sheet or arced structure to form a plurality of shim
elements.
[0005] In still another aspect, an imaging apparatus for producing
Magnetic Resonance (MR) images of a subject is provided. The
apparatus has a magnet assembly for producing a static magnetic
field B0 and a gradient coil assembly disposed within the magnet
assembly for generating a magnetic field gradient for use in
producing MR images, the apparatus includes a MRI shim element
including a plurality of thin wires extending substantially
parallel to B0.
[0006] In still another aspect, an imaging apparatus for producing
Magnetic Resonance (MR) images of a subject is provided. The
apparatus has a magnet assembly for producing a static magnetic
field B0 and a gradient coil assembly disposed within the magnet
assembly for generating a magnetic field gradient for use in
producing MR images, the apparatus includes a MRI shim element
including a plurality of thin wires extending substantially
perpendicular to B0
DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 illustrates a magnetic resonance imaging (MRI)
system.
[0008] FIG. 2 illustrates two shim elements.
[0009] FIG. 3 illustrates that shim elements of different sizes can
be cut out or punched out from a prefabricated composite wire/epoxy
sheet.
DETAILED DESCRIPTION OF THE INVENTION
[0010] Herein described are methods and apparatus that provide a
simple design to control the eddy currents generated in the shim
elements during any gradient operational conditions.
[0011] As used herein, an element or step recited in the singular
and proceeded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly recited. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features.
[0012] FIG. 1 illustrates a simplified block diagram of a system 10
for producing images to which embodiments of the below described
shim elements of the present invention are applicable. Although a
bore-type magnet is illustrated in FIG. 1, the present invention is
equally applicable to open magnet systems and other known types of
MRI scanners. The MRI system could be, for example, a GE Signa MR
scanner available from GE Healthcare which is adapted as described
herein, although other systems could be used as well.
[0013] The operation of the MR system 10 is controlled from an
operator console 12, which includes a keyboard and control panel
and a display (not shown). The console 12 communicates with a
separate computer system 14 that enables an operator to control the
production and display of images. The computer system 14 includes a
number of modules, which communicate with each other through a
backplane. These include an image processor module, a CPU module,
and a memory module, known in the art as a frame buffer for storing
image data arrays. The computer system 14 is linked to a disk
storage or optical drive for storage of image data and programs,
and it communicates with a separate system control 16 through a
high speed serial link.
[0014] The system control 16 includes a set of modules connected
together by a backplane. These include a CPU module 18 and a pulse
generator module 20, which connects to the operator console 12
through a serial link. The system control 16 receives commands from
the operator, which indicate the scan sequence that is to be
performed. The pulse generator module 20 operates the system
components to carry out the desired scan sequence. It produces data
that indicate the timing, strength, and shape of the radio
frequency (RF) pulses which are to be produced, and the timing of
and length of the data acquisition window. The pulse generator
module 20 connects to a set of gradient amplifiers 22 comprising of
G.sub.x, G.sub.y, and G.sub.z amplifiers (not shown) to indicate
the timing and shape of the gradient pulses to be produced during
the scan.
[0015] The gradient waveforms produced by the pulse generator
module 20 are applied to the gradient amplifier system 22. Each
gradient amplifier excites a corresponding gradient coil in an
assembly generally designated as 24 to produce the magnetic field
gradients used for position encoding acquired signals. The gradient
coil assembly forms part of a magnet assembly 26 which includes a
polarizing magnet 28 and a whole-body RF coil 30. A volume 32 is
shown as the area within magnet assembly 26 for receiving a subject
34 and includes a patient bore. As used herein, the usable volume
of a MRI scanner is defined generally as the volume within volume
32 that is a contiguous area inside the patient bore where the
homogeneity of the main, the gradient and the RF fields are within
known, acceptable ranges for imaging.
[0016] A transmitter module 36 in the system control 16 produces
pulses that are amplified by an RF amplifier 38 coupled to RF coil
30 by a transmit/receive module 40. The resulting signals radiated
by the excited nuclei in the subject 34 may be sensed by the same
RF coil 30 and coupled through the transmit/receive module 40 to a
preamplifier 42. The amplified MR signals are demodulated,
filtered, and digitized in a receiver 44. The transmit/receive
switch 40, is controlled by a signal from the pulse generator
module 20 to electrically couple the transmitter 36 to the RF coil
during the transmit mode and to connect the preamplifier 42 to the
RF coil during the receive mode.
[0017] The MR signals picked up by RF coil 30 are digitized by the
receiver module 44 and transferred to a memory module 46 in the
system control 16. When the scan is completed, an entire array of
data has been acquired in the memory module 16. An array processor
(not shown) operates to Fourier transform the data into an array of
image data. These image data are conveyed to the computer system 14
where they are stored. In response to commands received from the
operator console 12, these image data may be further processed by
an image processor within computer system 14 and conveyed to the
operator console 12 and subsequently displayed.
[0018] All commercial MRI systems employ passive shims to attain
the desired final homogeneity of the uniform magnet field B0. A
typical passive shim system includes a set of magnetized elements
of predetermined different volumes/denominations, which allows one
to create a desired magnetized moment on each shimming position.
The degree of quantization in such a system is determined by the
number of shim denominations and the volume of the smallest shim
element. Passive shims are positioned inside the bore or within the
gradient assembly, e.g. on the shim rails, in such a way that the
multitude of available shimming positions would provide an adequate
coverage of both axial and circumferential range, in order to
enable compensation of both axial and radial harmonics representing
B0 inhomogeneity. In European patent application EP 0677751, the
shim system employs shimming rings. Such rings due to their
symmetry can be used to compensate axial harmonics only, while the
need to compensate radial harmonics still remains. This need can be
addressed by a separate set of discrete shim elements which unlike
rings, do not have axisymmetric geometry and thus can allow
variation of their circumferential distribution. The shim elements
described below can be positioned on rails extending axially as
well as circumferentially, and therefore provide full complete
compensation for both axial and radial harmonics without resorting
to additional rings, thus reducing complexity. In the radial
direction, a stack-up of shim elements of different denominations
within maximum allowable radial thickness, provides the desired
total mass on each given position. System 10, in some embodiments,
includes shim elements as described below.
[0019] Herein described are shim elements that are composite and
made of a parallel alignment of multiple long ferromagnetic steel
wires held together by a filling material. The filling material can
be epoxy, rubber, plastic, etc. When the shim elements are
positioned on the rail, the wire orientation is typically parallel
to the B0 field of the magnet. However, the shim elements may be
turned such that the wires extend perpendicular to B0 and have a
reduced magnetization. Additionally, the shim elements can be at
any angle between being parallel and perpendicular to B0 in order
to achieve various shimming effects. Different sized shim elements
would be employed to provide the desired quantization.
[0020] FIG. 2 illustrates two different shim element designs; a
prior art shim 50 and a new shim element 60. The main magnet field
B0 and gradient magnet field components are also shown in FIG. 2.
By reducing wire cross-sectional dimensions a, and b to be below
the skin depth .delta..sub.sk, one can reduce eddy currents caused
by either dB.sub.r.sup.grad/dt, dB.sub.j.sup.grad/dt, or
dB.sub.z.sup.grad/dt to any predetermined level. This is
illustrated in FIG. 2 portions a) and b). The skin depth of the
shim material equals to .delta..sub.sk= {square root over
(2.rho./.mu..mu..sub.0.omega.)}, where .rho. is the material
resistivity, .mu. is the material relative permeability, .mu..sub.0
is the permeability of vacuum, and .omega. is the gradient circular
or angular frequency. In a saturated state inside the magnet, .mu.
is close to one. With 1006 steel, for example, .delta..sub.sk=6.6
mm at 1 kHz gradient frequency and is 21 mm at 100 Hz. In the
conventional shim piece 50 where dimensions are much greater than
.delta..sub.sk, (as in FIG. 2a) eddy currents are flowing around
the outer perimeter. In the herein provided configuration,
rectangular or round wires 62 have dimensions less than
.delta..sub.sk, and very limited or no contact with each other. In
some embodiments, the dimensions are much less than .delta..sub.sk.
For example, some embodiments use dimensions of 1/4 .delta..sub.sk
and 1/3 .delta..sub.sk. The eddy currents flow as indicated by an
arrow in FIG. 2b. The resultant heat generation in each wire is
proportional to d.sup.3 where d is the dimension normal to the
magnetic flux from the gradients. Since the number of wires per
piece (in each direction) increases as 1/d with smaller d, the
total heat generation per piece is decreasing with wire diameter as
proportional to d.sup.2. A central opening 64 in the shim elements
allow for easy mounting on a stud or screw. However, the central
opening is not necessary, and the shim elements may be mounted in
other ways. For example, the rail may include a pocket sized to
receive a shim element.
[0021] FIG. 3 illustrates that shim elements 60 of different sizes
can be cut out or punched out from a prefabricated composite
wire/epoxy sheet 70. In one example, the wires 62, either round or
rectangular, are laid out and impregnated with a non-conductive
bonding agent or filler material, such as, for example, epoxy.
Additionally, in another embodiment, rather than constructing a
flat sheet, the wires are wound around a drum or other jig and then
the non-conductive bonding agent is applied. The arced structure
from the winding around the drum may be flattened (bent straight)
or left in an arc resulting in an arced shim element. In the
embodiment with round wire, one advantage and technical effect is
lower cost. The wires are either bare (limited linear contact), or
pre-coated with varnish (no contact) and than impregnated by epoxy.
The maximum attainable volumetric ratio (bare wire) is .pi./2
{square root over (3)}, or 90.7%.
[0022] In the embodiment with rectangular wire, one advantage and
technical effect is higher volumetric magnetization (filling
factor). Wires can be separated (either sleeved or interlaid e.g.
by glass cloth or other non conductive material) prior to the
impregnation. Additionally, multiple shim elements may be stacked
in multiple layers either aligned as shown in FIG. 2b, or offset as
shown in FIG. 3.
[0023] Exemplary embodiments are described above in detail. The
assemblies and methods are not limited to the specific embodiments
described herein, but rather, components of each assembly and/or
method may be utilized independently and separately from other
components described herein.
[0024] While the invention has been described in terms of various
specific embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the claims.
* * * * *